Spark plug reading is inspecting a used plug's insulator, ground strap and threads under magnification to judge air/fuel mixture, ignition timing and heat range from the colors and marks combustion leaves behind.
Sunoco Race Fuels lays out what you are looking for: light tan to grayish on the insulator means the mixture and heat range are right; white or blistered means lean and about to hurt something; black and sooty means rich; a speckled, peppered look means detonation, so pull timing or add octane. The heat mark on the ground strap should sit roughly halfway along its length on a well-tuned race engine, and the fuel ring on the threads tells you whether the heat range fits. Method matters as much as the read: fresh plugs, a full-throttle run at your target RPM, then a clean kill with no idle time before you pull them. Use a loupe and good light, photograph each one, and compare cylinder to cylinder.
A plug read after a hard run is the cheapest early warning you will ever get for a lean cylinder, and a lean cylinder is how pistons die.
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The Weekly Race Car Maintenance Checklist That Reduces Preventable DNFs